This invention relates to a liquid crystal display device and, in particular, to a liquid crystal display device which utilizes a metal-insulator-metal (MIM) element as a switching element.
The methods currently used for display of images in liquid crystal displays are broadly classified as the simple matrix method and the active matrix method. The simple matrix method includes a liquid crystal interposed between two opposed, perpendicularly-intersecting sets of multiple ribbon-like electrodes. A drive circuit is connected to each of the ribbon-like electrodes. While this method is inexpensive because of the simple structure, the resulting picture contrast is less than satisfactory due to cross-talk.
In contrast, the active matrix method utilizes switches which are coupled to each of the individual picture elements and, thus, permits voltage retention. Because the selected voltage is retained even during the course of timeshared driving, the resulting large display capacity provides excellent image qualities such as contrast. The active matrix method nevertheless has a complicated structure and so the production cost is high. For example, thin film transistors (TFTs) are one such active element, and in their manufacture, five or more photomasks must be used to improve the yield for stacking five or six layers of thin film. Accordingly, two-terminal elements which allow improved yield and reduced production costs have been used in favor of other active elements.
The metal-insulator-metal element (MIM) is representative of these two-terminal elements. The general structure of the MIM element is shown in FIGS. 2 and 3. The insulating films in conventional MIM elements are TaO.sub.x formed by anodic oxidation of a lower electrode. Since the relative dielectric constant is about 30, the element capacitance is as high as 0.1 pF when the size of the element is approximately 5 .mu.m.times.4 .mu.m and the anodic oxide film thickness is approximately 600 .ANG.. This element capacitance is at least more than one-half of the liquid crystal capacitance per picture element.
Conventional MIM elements have three disadvantages. First, voltages applied to the liquid crystal panel are not fully applied to the MIM element because the capacity ratio of the capacitance of the liquid crystal to the MIM elements is 2 or less, and usually about 1.6. Thus, the switching property is inferior and the display quality of the liquid crystal panel is inferior to the quality of the TFT panel. One solution to this problem has been to reduce the size of the MIM element, such as to 3 .mu.m.times.3 .mu.m. However, this solution drastically reduces production yield. A second solution has been to increase the insulation film thickness, for example, to an anodized insulation film thickness of 1,000 .ANG.. However, although this reduces the capacitance of the MIM element, the non-linear coefficient (.beta.value) for the current-voltage of the MIM element is reduced, thereby negating the benefit of reduced capacitance.
The second disadvantage of conventional MIM elements is that the non-linear coefficient (.beta.value) of the I-V characteristic in the conventional MIM element is less than 3.4 even at room temperature, and falls to about 2.5 at higher temperatures around 60.degree. C. Moreover, the resistance of the MIM element in a low voltage area for R(3V) is as low as about 1.times.10.sup.11 ohm at room temperature and about 5.times.10.sup.9 ohm at 60.degree. C.
The third disadvantage of conventional MIM elements is that picture element division is impossible for MIM elements because of their low capacitance. In contrast, TFTs have been adapted in recent years to allow such redundant designs and, thus, their production yield has been improved. Moreover, the TFT panel with this improved production yield is similar in cost to the conventional MIM element.
Accordingly, it is desirable to provide a liquid crystal device which utilizes a new MIM element which has increased capacitance, increased .beta. value, decreased device resistance in the low voltage area and various temperatures, which allows for redundant design such as in picture element division, at a production cost comparable or less than the TFT panel.